A typical rotary joint consists of a fixed collimator holder and a rotatable collimator holder which are relatively rotatable each other to allow uninterrupted transmission of electro-magnetic signals through the rotational interface from collimators on any one of the holders to the collimators on another holder.
The multi-channel fiber optic rotary joints of typically utilize a de-rotating mechanism between the fixed collimator holder and the rotatable collimator holder. The optic de-rotating mechanism can be Dove prism, Delta prism, Abbe-Konig prism, and Schmidt-Pechan prism, which rotates at half the speed of rotation of the rotatable fiber collimator holder.
The examples of the prior arts include U.S. Pat. No. 4,109,998 (Dove prism), U.S. Pat. Nos. 4,460,242, 5,271,076 (Dove prism), U.S. Pat. No. 7,373,041 (Dove prism & Abbe-Konig prism) and US 2007/0019908 (Schmidt-Pechan prism & Abbe-Konig prism).
U.S. Pat. No. 4,109,998 rotary joint utilizes Dove prism as a de-rotation mechanism to de-rotate the images of an input set of optic transmitters located on the rotor, so that they may be focused onto stationary photo detectors located on the stator. De-rotation is accomplished by gearing the rotor and the prism in such a way that the prism rotates half as fast as the rotor. The optical rotary joint in U.S. Pat. No. 4,109,998 utilize light emitting diodes (LIDS) or lasers and laser detectors instead of optic fibers. As a result, it does not require the high alignment accuracy required for optic fibers, because the detectors may be quite large. The device is not bidirectional.
U.S. Pat. No. 4,460,242 discloses an optic slip ring employing optical fibers to allow light signals applied to any one or all of a number of inputs to be reproduced at a corresponding number of outputs of the slip ring in a continuous manner. It includes a rotatable output member, a stationary input member and a second rotatable member which is rotated at half the speed of the output member like a de-rotator. The input member having a plurality of equi-spaced light inputs and the output member having a corresponding number of light outputs and the second rotatable member having a coherent strip formed of a plurality of bundles of optical fibers for transmitting light from the light inputs on the input member to the light outputs.
Most of the prior arts with de-rotating mechanisms can only be used in air because fluids, having similar index of refraction to glass, would render the de-rotating mechanisms, such as a Dove Prism, useless. Additional they are limited to use in optics or the visible part of the electro-magnetic spectrum.
Metamaterials or left handed materials have long been known to be theoretically possible but do not occur naturally and have only been realized recently. These composite materials are engineered to have a negative index of refraction and can be manufactured for any desired frequency range over the entire electro-magnetic spectrum. They are so called left handed materials because rather than refracting light across the normal, like naturally occurring right handed materials, these materials refract light on the same side of the normal. This property is critical in to reducing the size of the de-rotating mechanism.